1. Field of the Invention
The present invention relates generally to a plasma jet ignition system for an internal combustion engine with particular but not exclusive application to an automobile, and more specifically to a plasma jet ignition system comprising two ignition energy sources, one for a spark ignition and the other for a plasma jet ignition, and a plasma jet spark plug which receives ignition energy from the two energy sources and performs a plasma jet ignition as well as a spark ignition.
2. Description of the Prior Art
A plasma jet spark plug for a plasma jet ignition system has two electrodes defining therebetween a spark gap and an insulating body surrounding the spark gap to form a discharge cavity of a small volume, and is provided with ignition energy from two energy sources. A spark discharge is produced between the spark gap of the plug by applying the ignition energy from a first energy source to the plug. A second energy source then supplies the ignition energy to the plug to maintain the spark discharge, thereby to produce in the discharge cavity a plasma gas of high energy, which is ejected through a spout orifice of the discharge cavity to ignite the combustible mixture.
It is known that a plasma jet ignition provides a complete and stable combustion of the combustible mixture in the combustion chamber in an engine, resulting in lower harmful engine emissions and in improvement of fuel economy. Thus a plasma jet ignition system provides a satisfactory engine performance with reliable ignition and stable combustion even at low engine load and at lean air fuel mixture in which, otherwise, poor ignition and misfire often occur. Furthermore, a plasma jet ignition system can start a cold engine very efficiently, even through fuel evaporation is so slow that the engine receives only a lean fuel mixture.
However, such a plasma jet ignition system requires a very high ignition energy, and a plasma jet spark plug must endure a very high temperature environment. A continuous high energy ignition, especially at high engine load or high engine speed, causes a rapid erosion of the electrodes of a plasma jet spark plug, and places so great an electric load on a battery and a charging system that a battery and an alternator of a large capacity are required.
Accordingly, there has been proposed an improved plasma jet ignition system which is arranged to decrease the ignition energy at high load or at high speed, where an acceptable combustion is easily obtained without a plasma jet ignition. However, such an improved system is still unsatisfactory in various ways. For example, such a system performs a plasma jet ignition during the engine cranking period, so that a plasma jet ignition together with engine cranking places an extremely large electric load on a battery. Furthermore, such a system operates in the same way whether the engine is cold or not. Therefore, the system does not provide a suitable amount of ignition energy as required in accordance with the engine temperature and results in engine operating difficulties. For example, insufficient ignition energy during cold start period causes a failure of cranking and extends the warm-up period, resulting in an increased amount of ignition power drain.